This invention relates to a method and system for setting a communication rate on a communications line.
When one modem wishes to communicate with another modem, a compatible communications rate must be found for both directions of communication. One technique used in determining a compatible communication rate is for a transmitting modem to send tones unique to transmitted data rates. Another technique is for a transmitting modem to send a pre-determined character which is determined prior to use of the system (e.g., the letter xe2x80x9caxe2x80x9d). The receiving modem oversamples the received waveform and, knowing that it represents the agreed character, can determine the transmission rate. It is also known for a receiving modem to send error detection information back to a transmitting modem. In such case, if an error rate is too high, the communications rate is reduced.
This invention seeks to provide an efficient approach to communications rate setting.
According to the present invention, there is provided a method of setting a downstream communications rate on a communications line, comprising the steps of: where no downstream communications rate is established on said communications line, establishing a base downstream communications rate on said communications line; determining a downstream signal to noise ratio (SNR) on said communications line; based on said downstream SNR, predicting an acceptable downstream communications rate; if said predicted acceptable downstream communications rate is greater than said established downstream communications rate, sending an indication of a new downstream communications rate on said line based on said predicted downstream communications rate and thereafter attempting to establish said new downstream communications rate on said line.
According to another aspect of the present invention, there is provided a method of setting an upstream communications rate on a communications line, comprising the steps of: where said communications line is silent in a downstream direction, sending energy pulses upstream and monitoring for transmissions downstream at a pre-determined downstream base communications rate; on detecting downstream transmissions at said pre-determined downstream base communications rate, synchronising to said downstream transmissions; processing each downstream transmission for a message comprising an upstream communication rate and, thereafter, transmitting upstream at said upstream communications rate.
In accordance with yet another aspect of the present invention, there is provided a system for setting a communications rate on a communications line, comprising, at an upstream end of said communications line: means, where no downstream communications rate is established on said communications line, for establishing a base downstream communications rate on said communications line; upstream means for determining a downstream signal to noise ratio (SNR) on said communications line; upstream means, based on said downstream SNR, for predicting an acceptable downstream communications rate; upstream means, if said predicted acceptable downstream communications rate is greater than said established downstream communications rate, for sending a message downstream indicating a new downstream communications rate on said line based on said predicted downstream communications rate and thereafter attempting to establish said new downstream communications rate on said line.